I wonder if that kit was recently released? I'm asking because I just checked the websites of three forum vendors that carry AP and none of them have the 15" (380 mm) kit listed.
Before committing to the larger kit you should check on the price of replacement rotors and pads. If the kit was designed properly it should use a slightly different pad shape (larger inner radius on the pad profile) to accommodate the larger rotor - pad price might not be significantly different but the rotor price will most likely be more expensive.
One other thing you should factor into your decision is whether 18" wheels will clear the 15" rotor.